feat: mentions and following tickets

Mention project members in comments with @, follow and unfollow tickets,
and get told when something changes on the tickets you follow. Commenting
on a ticket starts following it.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

libs/api/tracker/application/src/lib/event-handlers/event-handlers.ts

@@ -27,6 +27,7 @@ import { ReleaseDeletedEventHandler } from './release-deleted.event-handler';
 import { IssueAttachmentAddedEventHandler } from './issue-attachment-added.event-handler';
 import { IssueAttachmentRemovedEventHandler } from './issue-attachment-removed.event-handler';
 import { AttachmentCleanupEventHandler } from './attachment-cleanup.event-handler';
+import { IssueWatchersEventHandler } from './issue-watchers.event-handler';
 
 /** Read-model projections (MongoDB) + realtime, fed by the EventsBridge. */
 export const EVENT_HANDLERS = [
@@ -58,6 +59,7 @@ export const EVENT_HANDLERS = [
   IssueAttachmentAddedEventHandler,
   IssueAttachmentRemovedEventHandler,
   AttachmentCleanupEventHandler,
+  IssueWatchersEventHandler,
 ];
 
 export { ReadModelNotifier };